Chalmers University finds more effective fuel cell catalyst
July 3, 2017
A yttrium-platinum nano-alloy has been shown to be ten times as effective as pure platinum in a fuel cell
Chalmers University of Technology and Technical University of Denmark have been investigating replacement materials for platinum catalysts in fuel cells to reduce the need for the expensive element. New research into nano-alloys has shown that it is possible to reduce the platinum requirement significantly, and that the new technology is also well suited for mass production.
In this case, yttrium was bonded with the traditional platinum catalyst to create an alloy more effective than the single element alone. “A nano-solution is needed to mass-produce resource-efficient catalysts for fuel cells. With our method, only one tenth as much platinum is needed for the most demanding reactions. This can reduce the amount of platinum required for a fuel cell by about 70%”, says Björn Wickman, an Assistant Professor at the Department of Physics at Chalmers University of Technology.
Speaking to InnovOil by email, Wickman explained why the new material had proved to be more effective. “The Pt3Y alloy has the property that the oxygen reduction reaction (ORR) is much more efficient here than on pure Pt. The specific activity (surface activity) of Pt3Y is about a factor of seven higher than for pure Pt.”
In a fuel cell, he said, the most important figure is the mass activity – the current generated per mass of catalyst. “What we have done in this study is to present a method that can produce the Pt3Y as a nanomaterial, which is absolutely necessary in order to reach high mass activities. We show that our nanofilms of Pt3Y have a mass activity up to 10 times higher than nanoparticles of pure platinum. That means that if our material can be introduced in a real fuel cell only one tenth of the amount of the element would be needed on the cathode side, where the ORR takes place.”
If this level of efficiency can be achieved in a functional fuel cell, they could be produced using about as much platinum as an ordinary car catalytic converter. With platinum selling for around US$30 per gram and with 30-40 grams needed in a typical fuel cell, the solution could save as much as US$1,000 per unit, and help to push fuel cell vehicles to a cost-competitive target of around US$30 per kW of capacity.
Alloyed forces Previous research has proved that it is possible to mix platinum with other metals – in this case yttrium – to reduce the amount needed to produce a functional fuel cell. Yet no one has yet managed to create alloys with these metals in nanoparticle form in a manner that would be suitable for large-scale production. The biggest barrier to this is that yttrium tends to oxidise instead of forming an alloy with the platinum. Chalmers researchers have overcome this problem by combining the metals in a vacuum chamber, using a deposition technique called sputtering – where a mass of yttrium is bombarded with ions, causing it to eject particles which then bond with the platinum.
The result is a nanometre-thin film of the new alloy that allows mass-produced platinum and yttrium fuel cell catalysts. According to an abstract of the team’s paper, published in the Advanced Materials Interfaces journal: “The films show an improvement in stability over the same materials in nanoparticulate form. Physical characterisation shows that the thin films form a platinum overlayer supported on an underlying alloy. The high activity is likely related to compressive strain in that overlayer.” The successful demonstration of the technique is a major part of the team’s breakthrough. Traditionally, deposition of catalysts is done via a wet chemical method, but “in our case,” Wickman said, “There is no functioning wet chemical method to produce Pt3Y nanoparticles, despite much work during the past 10 years.”
They are also confident that the technique would be scalable to larger production volumes, although it would be more costly. Yet with the prospect of only using a tenth as much platinum, he believes the more expensive fabrication method could be justified.
Wickman says the next steps will be to test these catalysts in real cells. This will require some slight modification to the design of the fuel cell itself, largely because current electrodes produced using the chemical deposition method would not be reproducible using the sputtering technique.
“This might, however, not be as difficult as one might think,” he explained. “There has been a lot of research into alternative electrode designs – for example electrodes typically referred to as nanostructured thin films (NSTF). These have been identified as promising for future fuel cells and a large amount of research is now devoted to developing them. Most NSTF designs would be well suited to combine with sputtering to deposit the catalyst material.”
Commercial platinum-catalysed PEM fuel cells may have been around for over 60 years, but more innovation will be needed if they are to make it in mass-market applications. Nano-alloys such as these are but one step on the road to a hydrogen future.